Objective: To identify risk factors for adverse psychological outcomes among adult siblings of long-term survivors of childhood cancer. Methods: Cross-sectional, self-report data from 3083 adult siblings (mean age 29 years, range 18-56 years) of 5 + year survivors of childhood cancer were analyzed to assess psychological outcomes as measured by the Brief Symptom Inventory-18 (BSI-18). Sociodemographic and health data, reported by both the siblings and their matched cancer survivors, were explored as risk factors for adverse sibling psychological outcomes through multivariable logistic regression. Results: Self-reported symptoms of psychological distress, as measured by the global severity index of the BSI-18, were reported by 3.8% of the sibling sample. Less than 1.5% of siblings reported elevated scores on two or more of the subscales of the BSI-18. Risk factors for sibling depression included having a survivor brother (OR 2.22, 95% CI 1.42-3.55), and having a survivor with impaired general health (OR 2.15, 95% CI 1.18-3.78). Siblings who were younger than the survivor reported increased global psychological distress (OR 1.81, 95% CI 1.05-3.12), as did siblings of survivors reporting global psychological distress (OR 2.32, 95% CI 1.08-4.59). Siblings of sarcoma survivors reported more somatization than did siblings of leukemia survivors (OR 2.07, 95% CI 1.05-3.98). Conclusions: These findings suggest that siblings of long-term childhood cancer survivors are psychologically healthy in general. There are, however, small subgroups of siblings at risk for long-term psychological impairment who may benefit from preventive risk-reduction strategies during childhood while their sibling with cancer is undergoing treatment.